Charcoal kiln

Equipment to apply heat to wood or other cellulose sources to chemically convert the cellulose to charcoal. Like a lime kiln they can be a pure simple kiln or a damped pyre.

Yield

Charcoal is both smaller (~35%) and lighter (10%) than the wood it comes from.

Volume

3.6m³ of dry wood becomes 1-1.4m³ of charcoal.

Mass

10kg dry wood becomes 1kg of charcoal.

Types

Airtight, usually behive-shaped, brick constructions with a loading port at ground level and a topping/lighting port in the center of the dome. Can range from a few feet to a few stories in height. By far the most effort to produce and subject to the same batch-size restrictions as a pit kiln, it's easier to load and unload and more resistant to environmental damage. It's also far safer to fire than the either of the other methods.

Usage

The structure is loaded as tightly as possible with wood, usually surrounding a small (6-12 inch) vertical central column of air or quick-burning tinder that stretches from the floor to the lighting port. The central column is lit and the topping port left open until the fire is well established at the very bottom of the stack. Then the topping port constricted and the lower port (if any) is closed and sealed, allowing little or no air into the kiln. The resultant process is a self-sufficient exothermic smouldering of the wood. When the smouldering process eventually stops, the structure is opened and emptied. Some coals will still be live, and they are separated from the hot, but not burning, charcoal.

The charcoal is the primary product. It is stored and kept for later use. The coals can be used in a different fire, or smothered and reused. The water vapor hopefully escapes during the first part of the smouldering process (before the top port is closed). The wood distillates will either escape with the water vapor, or they can be ducted and condensed for use. The wood tar accumulates on the floor, and while it has some limited uses (waterproofing) it is mostly a waste product. It should be regularly cleaned off the floor because it invites flareups (which cause the wood to burn to ash, rather than smoulder to charcoal) and poisons the charcoal it coats, typically smaller pieces that accumulate on the floor.

Hazards

This process involves raising wood far beyond its flashpoint and restricting oxygen to it to cause it to outgas and char rather than burn. It also produces flammable, toxic and caustic wood distillates and superheated steam. These are inherently dangerous processes and byproducts. Treat this fire-related process with substantial caution.